First of all thanks for everybody who contributes to this forum, it's a great source for learning.

I've started with arduino last week and managed to get a serial 7-digit display, oled and rtc clock running. I have a question about the next step:

I'm making a portable device using the arduino to control the oled and take input from buttons, once I'm happy with the prototype on a breadboard:

1 - I'm using a arduino mega, will the same code work for the arduino pro mini?2 - do I actually need an arduino board or could I order a custom PCB that I could solder the atmega (or attiny, etc) and the other components (resistors, voltage reg., etc)?

I won't need to conect the device to a computer or anything, it'll run on batteries.